Cancer risk in populations exposed to ionizing radiation are evaluated by dose, radiation quality, fractionation, time, sex, age at exposure and at observation, and modifying influences of other environmental and host factors. Models of radiation carcinogenesis are formulated and tested to help define basic mechanisms. Populations studied include Japanese A-bomb survivors, patients given therapeutic or diagnostic radiation, and occupational groups. Environmental exposures such as residential radon are also studied. Program members serve on committees advising the government and international agencies. Results of studies suggest that (1) living near nuclear facilities in the United States is not associated with a detectable increased risk of childhood leukemia or any other cancer; (2) approximately 10% of all lung cancer deaths may be due to indoor radon; however, a detectable risk at residential levels was not found in a large-scale study in China; (3) low-dose diagnostic x-ray exposures during adolescence to monitor scoliosis may pose some future risk of breast cancer; (4) diagnostic x-rays may not be causally related to leukemia or lymphoma but simply related to conditions that portend their development; (5) susceptibility to radiogenic breast cancer declines with increasing age at exposure, the dose response is linear, and risk remains for at least 50 years; (6) children irradiated for benign conditions are at risk of developing thyroid and skin cancers; (7) low-dose radiotherapy to treat uterine bleeding induces many more leukemias than high-dose radiotherapy to treat cervical cancer; (8) high-dose radioactive iodine treatments did not increase the risk of leukemia or other cancer, suggesting that protraction of dose reduces risk; (9) similarly, high natural background radiation does not appear to increase the risk of nodular thyroid disease.